1. Field of the Invention
The present invention relates to a progressive power lens used to assist an accommodation power of an eye.
2. Related Background Art
As spectacle lenses for assisting an accommodation power when an accommodation power of an eye becomes too weak to see a near point, various progressive power lenses have been known. Such a progressive power lens has a distant vision correcting region (referred to as a "portion for distant vision" hereinafter), a lower near vision correcting region (referred to as a "portion for near vision" hereinafter) and a progressive region (referred to as an "intermediate portion" hereinafter) located between the above upper and lower portions in which a refracting power is continuously changed. When worn by a user, the portion for distant vision is located above the portion for near vision. In the present invention, it is to be noted that "upper", "lower", "horizontal" and "vertical" are used to refer to positions of the progressive power lens in the state that it is actually worn by a user. For example, the lower portion of the portion for distant vision is a position within the portion for distant vision close to the intermediate portion. Also, the difference between the refracting power of the portion for near vision and the refracting power of the portion for distant vision is called an additional power.
In a progressive power lens of this type, in general, when wide clear vision areas of the portions for near and distant vision are kept and coupled by the progressive power range, lens aberrations are concentrated on side areas of the progressive power range. As a result, blurring and distortion of an image occur in these side areas. Therefore, when a user wears spectacles formed of these power lenses and shifts his eyes laterally, he perceives the distortion of an image as the fluctuation of the image and has an unpleasant feeling.
In order to solve such a problem of visual characteristics, in known progressive power lenses, various designs and evaluations have been made from a variety of viewpoints.
FIG. 1 is a schematic plan view showing the divisions of regions of a progressive power lens designed symmetrically. The progressive power lens has, in the state with it worn by a user, an upper portion F for distant vision, a lower portion N for near vision and an intermediate portion P in which the refracting power is changed continuously between the upper and lower portions F and N. As for the shape of a lens surface, the intersecting curve MM' of the object-side lens surface and a cross section along a meridian line passing through approximately the center of the lens surface vertically from top to bottom is used as a reference line for expressing specifications such as the additional power of the lens, and is also used as an important reference line in the lens design. In the thus designed progressive power lens, the center OF of the portion F for distant vision, an eye point E for distant vision and the center (eye point) ON of the portion N for near vision are located on the center line MM' as the reference.
Further, as shown in FIG. 2 (later discussed in connection with the invention), in consideration of the fact that the portion N for near vision comes close to a nasal side when worn by a user, another progressive power lens has a portion N for near vision and an intermediate portion P arranged asymmetrically (hereinafter referred to as an "asymmetrical type progressive power lens").
Also, in such an asymmetrical type progressive power lens, a center line MM' consisting of the intersecting curve of an object-side lens surface and a cross section passing through a center OF of a portion F for distant vision, an eye point E for distant vision and a center ON of a portion N for near vision is used as a reference line. In the present invention, these reference lines will be referred to as "principal meridional curves" hereinafter.
Thus, in the symmetrically designed progressive power lens (hereinafter referred to as the symmetrical type progressive power lens), the principal meridional curve MM' divides the lens surface symmetrically into a nasal side region and a temporal side region. On the other hand, in the asymmetrical type progressive power lens, the principal meridional curve MM' is displaced toward a nasal side in the intermediate portion P and the portion N for near vision.
FIG. 3 shows an astigmatic difference distribution of a known symmetrical type progressive power lens. Isoastigmatic difference curves in FIG. 3 are curves formed by linking points which have astigmatic differences of 0.50 diopter. An area including a principal meridional curve sandwiched by the isoastigmatic difference curves is a clear vision area.
In general, for looking at an object with both eyes by the use of such symmetrical type progressive power lenses as spectacles, the lenses are rotated by about 8.degree. to 10.degree., as shown in FIG. 4, in consideration of the fact that both eyes are converged in accordance with movement of the line of sight from a distant view to a close-range view. Specifically, when setting the lenses in a frame of the spectacles in a manner as indicated by broken lines in the drawing, the principal meridional curve MM' of the lens on the right eye side is inclined counterclockwide at the angle of about 8.degree. to 10.degree. with respect to the vertical direction in the worn condition while that of the lens on the left eye side is inclined clockwise at the same angle with respect to the same direction.
As a result, when looking at an object with both eyes, the right and left eyes look at the object through areas having different astigmatic differences. Therefore, for example, when both eyes look toward the right side at infinity, the left eye looks through an area having a larger astigmatic difference as compared to the right eye, so that the user feels uncomfortable.
More specifically, with reference to FIG. 4, when both eyes look at the right side at infinity, positions through which the lines of sight of the right and left eyes pass are points A and B respectively. The points A and B are on a horizontal line C indicated by a broken line. As mentioned above, the meridional lines MM' of the lenses on the right and left eye sides are inclined counterclockwise and clockwise respectively, so that the point A is located within the clear vision area while the point B is located outside that. Thus, since the left eye looks at the object through the area having the larger astigmatic difference as compared to the right eye, it is preferable to lessen the astigmatic difference in that area.
Therefore, in order to avoid uncomfortable feelings in the worn state, various asymmetrical type progressive power lenses have been proposed, in which a principal meridional curve is displaced toward the nasal side in an intermediate portion and a portion for near vision to secure the symmetry of an aberration distribution in the horizontal direction in the worn state thereby to improve visual performance. Such asymmetrical type progressive power lenses are disclosed in, e.g., Japanese Patent Publication Nos. 47-9626, 2-39767 and 64-5682.
In these conventional asymmetrical type progressive power lenses, improvement of the visual performance can be achieved to some extent but cannot be achieved sufficiently.
Namely, in the lens disclosed in Japanese Patent Publication No. 47-9626, the astigmatic difference distribution is symmetrical in the horizontal direction in the worn state, but lens refracting surfaces of portions for distant and near vision are spherical, so that it is impossible to widen the width of the clear vision area of an intermediate portion connecting the portions for distant and near vision smoothly.
In the lens disclosed in Japanese Patent Publication No. 64-5682, in consideration of movement of the lines of sight when both eyes look at an object in the state with the lenses worn, the astigmatic difference distribution is made symmetrical in the horizontal direction within 15 mm from the principal meridional curve respectively toward the left and right sides in an area where the principal meridional curve is displaced toward the nasal side. Accordingly, with respect to movement of the lines of sight at the time of looking at an object, the absolute values of the astigmatic differences to the respective left and right eyes become approximately equal. However, the density of the astigmatic differences becomes low on the temporal side region while it becomes high on the nasal side region. As a result, fluctuation and distortion of an image are felt much more in the side areas of the intermediate portion close to the clear vision area thereof where the density of the astigmatic differences is highest.
In the lens disclosed in Japanese Patent Publication No. 2-39767, the astigmatic differences at a pair of points spaced the same distance away in the horizontal or vertical direction from an arbitrary point on the principal meridional line are made approximately equal over the entire lens refracting surface. However, since the widths of the clear vision areas of portions for distant and near vision are made large, the width of the clear vision area of an intermediate portion is narrowed and the value of the maximum astigmatic difference is enlarged. In the above conventional asymmetrical type progressive power lenses, visual performance is secured to some extent but is not yet sufficient for practical use.